This invention is directed to an integral internal circulation system for a fluid bed biological or adsorption reactor.
Fluid bed reactors have been demonstrated as effective supplemental or replacement reactors for adsorptive or biological treatment of waste water for BOD removal, nitrification and denitrification. Fluid bed reactors, as stated, may be biological or adsorptive in character. In biological reactors, the treatment is accomplished primarily by microorganisms which feed on the nutrients in the wastewater. In adsorptive reactors the treatment is primarily physical with the impurities in the wastewater being adsorbed and so entrapped on an appropriate medium such as activated carbon.
The basic apparatus and methods of operation of fluid bed biological reactors are disclosed and covered in a series of patents including U.S. Pat. No. 3,846,289, issued Nov. 5, 1974, U.S. Pat. No. 3,956,129, issued May 11, 1976, U.S. Pat. No. 4,009,099, issued Feb. 22, 1977 and U.S. Pat. No. 4,009,098, issued Feb. 22, 1977.
These patents disclose that, in a fluidized bed environment, where solid particles, such as sand, form a bed which is suspended in an upwardly flowing liquid stream with the particles in continuous motion, an enormous surface area for biological growth is available. When appropriate conditions of temperature, pH, availability of food, absence or presence of oxygen, are maintained, biological growth is remarkably rapid so that the reactor volume required and the retention time necessary to achieve a given biological conversion are drastically reduced. Accordingly, then, substitution of fluid bed biological reactors for the reactors in common use today, gives promise that the land area now devoted to biological treatment plants, can, in the future, be significantly reduced.
In another patent, U.S. Pat. No. 4,202,774, issued May 13, 1980, it is pointed out that while fluidization and suspension of particulate solids are extremely important in promoting biological growth on the solids in fluid bed biological processes, excessive agitation of the solids is to be avoided to reduce abrasion of the solids with resultant removal of the biological growth. In order to avoid the deleterious effects of excessive agitation of the fluidized particulate solids, the patent discloses novel liquid flow distributors capable of introducing influent into the fluid bed reactor and directing large flow volumes vertically upward in the reactor without causing excessive turbulence therein.
In fluid bed waste water reactors of the type described above, a certain loading rate (mass loading) on the reactor is required to allow the desired biological reactions to occur, and this sets a limit on the volume of untreated waste water which can be introduced into a reactor of given size in a specified period of time. However, for this same reactor, there is a minimum flow of liquid into the reactor which must be observed to maintain the necessary degree of fluidization of the particulate solids. In order to satisfy both conditions, recirculation of the liquid from above the fluidized bed to the reactor liquid flow distributor is undertaken; the necessary flow rate for fluidization is thus achieved, while the influent can be limited to that flow which can be adequately treated in the reactor.
The recirculation circuit which has been used is a system external to the reactor and includes a pump, a conduit connecting the region of the reactor above the fluidized zone of the reactor with the pump for withdrawing liquid from that region and a conduit connecting the pump to a region below the fluidized zone of the reactor for reinjection of the liquid to maintain fluidization. The piping for such installations, with pipes often having a diameter of 24 inches or more, is expensive; the cost of external recirculation piping for a typical reactor representing 15 to 20% of the installed cost.
Accordingly, a very real need exists for an improved recirculation system for fluid bed reactors.
In the main, prior art structures, if at all relevant, are generally directed to sand filters rather than fluid bed reactors. In the process of sand filtration, the liquid to be filtered passes downwardly through a sand bed which removes particulate material from suspension in the liquid. This downflow system does not result in fluidization of the sand particles during normal operation. The following patents show various aspects of the prior art in sand filters:
______________________________________ U.S. Pat. Nos. ______________________________________ 1,138,634 J. M. Davidson May 1915 1,919,565 W. C. Laughlin July 1933 2,199,891 M. J. Martin May 1940 3,512,649 R. Nebolsine et al May 1970 3,625,365 J. E. Armstrong December 1971 ______________________________________
The following U.S. patent is directed to a reaction vessel for a catalytic gas-liquid reaction in which recycle of liquid, which separates from vapor in an upper chamber of the reactor, is provided:
______________________________________ U.S. Pat. No. 3,414,386 E. D. Mattix December 1968 ______________________________________